The present invention relates to camshaft phasers for internal combustion engines; more particularly, to schemes for controlling the action of such camshaft phasers; and most particularly, to a method and apparatus for controlling such action, including a novel target wheel for measuring the phaser instability within a camshaft revolution, and a means for compensating for shifts in measured instability.
Camshaft phasers for varying the valve timing of internal combustion engines are well known. A phaser typically comprises a rotor element attached to the end of a camshaft and variably displaceable rotationally within a stator element driven by the engine crankshaft. Phasers typically are actuated by pressurized oil derived from the engine""s main oil supply and selectively directed to chambers within the phaser to alter the phase relationship between the rotor and stator, and hence between the camshaft and crankshaft.
A torque-imposed instability is known in the art that can cause the phase relationship to vary from nominal during a 360xc2x0 rotational cycle of the camshaft. In opening an engine valve, the valve follower leaves the base circle portion of the camshaft lobe and begins to climb the rising edge of the eccentric portion, imposing a resistive (negative) torque on the camshaft. At some further position of camshaft rotation, the resistive torque reaches a maximum negative torque, then returns to zero, and then becomes an assistive (positive) torque as the follower descends the falling edge of the eccentric portion of the lobe, as the valve closes. This negative-positive fluctuation repeats itself during subsequent rotational cycles of the camshaft lobe. Because of mechanical and hydraulic lash in the system, and as a result of the fluctuating torque, the actual rotor position with respect to the stator may be significantly different from intended during valve opening and closing. The difference between the maximum negative and positive angular departures from nominal is known in the art as xe2x80x9cphaser instabilityxe2x80x9d. A typical cam phaser in good working order exhibits a characteristic and repeatable level of operational phaser instability due to the inherent mechanical and hydraulic lash in the system. However, an undesirable shift in the predictable level of phaser instability can result in sub-optimal valve phasing relative to crankshaft rotation.
What is needed is means for measuring the level of instability continually during engine operation, detecting when the level of instability changes, and causing the cam phaser and engine to take predetermined action when measured instability exceeds a predetermined threshold level.
It is a principal object of the present invention to detect when the level of instability in a camshaft phaser changes.
It is a further object of the invention to alarm such changes and to cause the phaser to take predetermined action to minimize potential problems such as engine malfunction and emissions increase.
Briefly described, a camshaft phaser control system in accordance with the invention includes a target wheel mounted on a phaser rotor which in turn is rotatable with the camshaft. Alternately, the target wheel may be machined in an end of the camshaft or otherwise fixed to the camshaft in known fashion. (For purposes of illustration, the target wheel described herein is mounted on the phaser rotor). As a reference point, the phaser control system also includes a means for detecting the rotational position of the crankshaft.
The target wheel is provided with first and second signal-initiating means, preferably in the form of trailing or falling edges of a first and second tooth on the target wheel, for measuring camshaft oscillatory instability. A first tooth is angularly placed with respect to one of the cam lobes such that the falling edge of the first tooth coincides with the peak excursion of the negative camshaft oscillation. The second tooth is angularly placed with respect to the same cam lobe such that the falling edge of the second tooth coincides with the peak excursion of the positive camshaft oscillation. Such peaks are readily determined via a torsion meter applied to a test engine, in known fashion.
The target wheel is mounted on the phaser rotor or on the camshaft such that, during camshaft rotation, the trailing edge of the first and second tooth initiates a signal to generate first and second signals in known fashion. In addition, equally spaced teeth are placed radially about the axis of rotation of the crankshaft to detect the rotational position of the crankshaft to serve as a reference point for the phaser control system.
The signals are transmitted to an electronic monitoring system which, in turn, by algorithm, measures phaser instability. This measurement is taken, for example, every camshaft rotation so that if a shift in phaser instability occurs, which can signify degraded phaser performance, the electronic monitoring system can take defensive actions.
Locating the teeth at the specific positions with respect to the cam lobe provides three important benefits. First, the system thus measures the maximum oscillatory instability in phaser performance, and therefore any increase in instability amplitude may be inferred as system malfunction. Second, such placement also maximizes the sensitivity of the system to such malfunction. Third, such placement makes the system least sensitive to changes in angular location of the peaks, which may shift as much as ten crank angle degrees with changes in engine speed.
The EMS is programmed in known fashion to change the duty cycle of the phaser, to limit phaser operation, or even to disable phasing, based on the magnitude of the instability. The system continues to monitor the level of instability. Should instability fall below the threshold limit, normal phasing operation is resumed.